There has been reduced to practice an inter-vehicle distance control device which measures the actual distance between the vehicle of a certain driver and a preceding vehicle by a driving environment sensor such as a laser instrument, determines a target inter-vehicle distance on the basis of the measured actual inter-vehicle distance and the speed of the vehicle of the driver, and controls the speed of the driver's vehicle in such a manner that the actual inter-vehicle distance may coincide with the target inter-vehicle distance.
Such an inter-vehicle distance control device controls the speed of the driver's vehicle so as to make the actual inter-vehicle distance coincident with the target inter-vehicle distance when the driving environment sensor detects the existence of a preceding vehicle, but controls the speed of the driver's vehicle so as to settle it to a preset speed when the driving environment sensor detects no preceding vehicle. However, since the preset speed is usually the upper limiting speed given by the control device installed on the driver's vehicle irrespective of whether there is a preceding vehicle or not, the driver's vehicle is necessarily accelerated to the preset speed whenever the driving environment sensor detects no preceding vehicle.
It often happens that the driving environment sensor fails to detect a human being or a physical object moving on the road such as a walking person or a running bicycle, or that it fails to correctly detect the road conditions such as sharp curves or intersections.
Accordingly, if such an inter-vehicle distance control device as described above is used on a vehicle driving along any road other than an automobile road dedicated to automobiles alone, such as a freeway or an express way, the vehicle may sometimes be accelerated even when a man or a bicycle is ahead of the vehicle, or even when the vehicle is entering a sharp curve or an intersection, provided that the driving environment sensor detects no preceding vehicle. For this reason, the inter-vehicle distance control device is so designed that it cannot be used on any road other than the road dedicated to automobiles alone.
As a result, in a driving environment where the inter-vehicle distance control device can be effectively used, it enables the driver to drive with less fatigue and higher security, but since it cannot be used in most driving environments, it hardly benefits the driver even if it, which is usually expensive, is installed on the driver's vehicle.
To overcome this shortcoming, there have been proposed methods with which the inter-vehicle distance control device can be used in as many driving environments as possible.
For example, Japanese Patent document JP-A-9-50597 proposes a method according to which the vehicle speed is automatically decelerated when the vehicle is entering a curve, by the help of a car navigation system recognizing the road condition. According to JP-A-9-50597, however, the car navigation system has not reached a practical level where it can be used to assist the inter-vehicle distance control device in recognizing the driving environment. For the GPS, which is incorporated in the car navigation system to locate the current position of the vehicle on which it is installed, does not have a precision high enough to correctly show the road on which the vehicle is traveling, or the maps included in the navigation system may possibly indicate road shapes different from the actual ones.
In addition, various researches have been made for the effective detection of a human being or a bicycle, but no practical method has been found yet.
The inter-vehicle distance control device is a device which automates the operations of both the acceleration pedal and the brake pedal, but a method has been proposed wherein only the operation of the brake pedal is automated so as to alleviate the burden on the driver as much as possible in such a situation as described above.
For example, Japanese Patent document JP-A-10-172100 discloses an inter-vehicle distance control device which detects the distance between the driver's vehicle and a preceding vehicle by using a radar and performs only deceleration control for decelerating the speed of the driver's vehicle on the basis of the detected inter-vehicle distance when there is no actuation of acceleration pedal by the driver. With this control device, the driver's vehicle can be prevented from being dangerously accelerated when a human being or a bicycle is ahead of it. This control device, therefore, can be used on a road other than express ways or freeways and can also contribute very much to the mitigation of the fatigue and the improvement in the security, of the driver.
Further, Japanese Patent document JP-A-10-329684 disclosed a method which serves to decrease the frequency of the driver's braking actions through the automatic application of the brake while there is no actuation of accelerator pedal by the driver.
However, according to the teaching of JP-A-10-172100, the driver's vehicle continues its free running while the radar does not detect any obstruction ahead. Consequently, the driver's braking action my be retarded in case where the radar fails to detect an obstacle, contrary to the driver's erroneous conviction that the radar can detect any obstacle and lead to braking action.
Further, the method proposed by JP-A-10-329684 has the following problem. Namely, the method works usefully when there is a vehicle running ahead of the driver's vehicle. However, when there is a preceding vehicle and also when the preceding vehicle is decelerated at a deceleration greater than the deceleration at which the driver's vehicle is automatically decelerated through automatic braking, the driver must additionally apply the brake. On the other hand, when the deceleration of the preceding vehicle is lower than the deceleration at which the driver's vehicle is automatically decelerated, the deceleration of the driver's vehicle must be controlled by alternating the accelerating action by stepping on the accelerator pedal and the decelerating action through the automatic braking.